Filamentous bacteriophages are a group of related viruses that infect bacteria. They are termed filamentous because they are long and thin particles comprised of an elongated capsule that envelopes the deoxyribonucleic acid (DNA) that forms the bacteriophage genome. The F pili filamentous bacteriophage (Ff phage) infect only gram-negative bacteria by specifically adsorbing to the tip of F pili, and include fd, f1 and M13.
The mature capsule of Ff phage is comprised of a coat of five phage-encoded gene products: cpVIII, the major coat protein product of gene VIII that forms the bulk of the capsule; and four minor coat proteins, cpIII and cpIV at one end of the capsule and cpVII and cpIX at the other end of the capsule. The length of the capsule is formed by 2500 to 3000 copies of cpVIII in an ordered helix array that forms the characteristic filament structure. About five copies each of the minor coat proteins are present at the ends of the capsule. The gene III-encoded protein (cpIII) is typically present in 4 to 6 copies at one end of the capsule and serves as the receptor for binding of the phage to its bacterial host in the initial phase of infection. For detailed reviews of Ff phage structure, see Rasched et al., Microbiol. Rev., 50:401-427 (1986); and Model et al., in "The orientation with the acidic amino terminus toward the outside and the basic carboxy terminus toward the inside of the membrane. The mature protein is about 50 amino acid residues in length of which 11 residues provide the carboxy terminus, 19 residues provide the hydrophobic transmembrane region, and the remaining residues comprise the amino terminus. Considerable research has been done on the secretion signal region of cpVIII to advance the study of membrane protein synthesis and targeting to membranes. However, little is known about the changes that are tolerated in the structure of the cpVIII membrane anchor region that would allow for assembly of phage particles.
Manipulation of the sequence of cpIII shows that the C-terminal 23 amino acid residue stretch of hydrophobic amino acids normally responsible for a membrane anchor function can be altered in a variety of ways and retain the capacity to associate with membranes. However, those anchor-modified cpIII proteins lost their ability to genetically complement gene III mutants indicating that the requirements of a membrane anchor for functional assembly have not been elucidated.
Ff phage-based expression vectors have been described in which the entire cpIII amino acid residue sequence was modified by insertion of short polypeptide "epitopes" [Parmely et al., Gene, 73:305-318 (1988); and Cwirla et al., Proc. Natl. Acad. Sci. U.S.A., 87:6378-6382 (1990)] or an amino acid residue sequence defining a single chain antibody domain. McCafferty et al., Science, 348:552-554 (1990). These hybrid proteins were synthesized and assembled onto phage particles in amounts of about 5 copies per particle, a density at which normal cpIII is usually found. However, these expressed fusion proteins include the entire cpIII amino acid residue sequence and do not suggest fusion proteins that utilize only the carboxy terminal membrane anchor domain of cpIII.
In addition, no expression system has been described in which a phage coat protein has been engineered to allow assembly of a heterodimeric molecule that is functional and capable of incorporation into the coat of a phage particle.